postulated that increased population size leads to increased social interaction. Among gregarious animals such increased social interaction enhances emotional involvement and elicits central activation necessary for sensory fixation and recognition even in emotionally neutral encounters. Thus, he postulates that every such stimulus contributes to the level of activation of both the brain-stem reticular formation and the major endocrine systems.
Welch's studies seem to indicate that one of the effects of increased population size and density is to increase the importance of the social environment as a determinant of physiological response to various stimuli, including the disease-producing agents to which the population is subjected. They suggest that the effect of any disease-producing agentóbe it a microorganism, a toxin, or some other physicochemical elementócannot be assessed without knowing the size and nature of the group within which the exposed population interacts and that the larger the interacting group, the more important will these group phenomena be in modifying the responses to disease-producing agents.
Such a formulation receives at least circumstantial support from studies on the level of blood pressure that have been conducted over the last 30 years in every continent in the world (19-43). These studies have indicated that, with few exceptions, populations living in small, cohesive societies tend to have low blood pressures which do not differ in the young and the aged. In a number of these studies, groups who have left such societies and have had contact with western urban culture were found to have higher levels of blood pressure and to exhibit the familiar relationship between age and blood pressure found in studies of western populations.
Disease, Population, and Categories of Individuals
This formulation, useful as it may be as a general proposition, requires further specification and modification if it is to determine future research strategy. Specifically, it seems important to recognize that the influences of increases in population size are going to vary for different categories of individuals. The first such category is determined by hierarchical position within the group. Welch (18) and Mason (44) have shown in animal experiments that the animals occupying subordinate positions within any group tend to respond in a far more extreme fashion to standardized stimuli than do those in dominant positions. Their responses include changes in endocrine secretions as well as manifestations of disease and pathology. To the best of my knowledge, no human studies have been conducted to test whether this particular phenomenon applies in humans, but there seem to be no a priori reasons to suspect that it does not.
For the second category there is both human and animal evidence. It concerns the degree to which the exposed populations have or have not been prepared by previous experience for the demands and expectations of the